Plasma arc torches are employed for cutting metal of various thicknesses. These plasma cutting torches involve an electrode within a cylindrical cavity of a nozzle having an outlet through which the plasma column issues after being established in the annular passageway between the electrode and nozzle. Electrical energy established by a voltage differential between the electrode and nozzle creates an arc in the nozzle for ionizing the gas before it passes from the outlet of the nozzle toward the workpiece, normally metal. The material being cut is conductive and has a variety of thicknesses. The plasma cutting process creates ionized gases within the annular chamber between the nozzle and electrode which establishes and maintains the plasma arc column due to the energy provided by the power supply connected between the nozzle and electrode and between the nozzle and the workpiece. A vortex action of the plasma gas passing through the annular chamber in the nozzle is caused by forcing the gas through a plurality of holes leading to the gas passageway. The swirling gas is focused in the nozzle chamber where it can be ionized and discharged through the small orifice or outlet of the nozzle. During cutting of relatively thin metal workpieces, the nozzle of the plasma torch is often dragged directly over and in contact with the metal. The torch is operated in a low current mode which maintains a current in the range of about 35-50 amperes, which is at a set point sufficient to sustain a stable arc and to obtain realistic traveling speeds and cutting quality. The low current mode of operation, with a current in the general neighborhood of 40 amperes, does not cause substantial nozzle damage even when the conductive nozzle is dragged along the conductive metal workpiece with the arc extending through the nozzle opening. The low current causes a lower energy and, thus, does not substantially overheat the nozzle. However, when cutting thick metal, the plasma torch is operated in a high current mode wherein the power supply is set at a current level over 75 amperes. With this high current flow necessary to sustain a focused arc plasma sufficiently powerful to cut a relatively thick workpiece, the nozzle itself must be maintained above the conductive workpiece so that the nozzle does not contact the workpiece and allow the arc to attach to the nozzle. In practice, an maintains the nozzle at least about 0.100-0.125 inches from the metal workpiece being cut by the moving plasma torch as the torch is operated in a high current mode. The relationship between the arc current supplied to the torch and the thickness of the metal being cut is generally set so that the ratio of the arc current amperes divided by 100 equals the thickness of the plate which can be cut. Thus, with approximately 40 amperes of arc current, which is considered the low current mode of operation, a metal plate of 0.40 can be cut efficiently by the moving plasma torch. The torch can be operated with the nozzle in contact with and being dragged along the workpiece. As the thickness of the workpiece increases to 0.750 inches, the arc current is set at the power supply to be in the high current mode, which mode exceeds about 75 amperes. The high current mode is generally above 40-50 amperes and preferably above about 25 amperes. In this high current mode of operation, it is essential that the nozzle not drag along or contact the workpiece. If the nozzle contacts the workpiece in the high current mode of operation, the life of the nozzle, and thus, the torch is drastically reduced. Thus, during the high current mode of operation, it is considered somewhat mandatory to maintain a certain spacing between the nozzle and the workpiece being cut. This requires manual dexterity skills, especially when the cut is to produce a bevel edge. In this instance, gaging the spacing of the torch from the workpiece becomes somewhat difficult.
When a plasma system is operated by an unskilled operator, it is not uncommon for the torch to be operated with the nozzle in contact with the workpiece. This does not present a problem if the thickness of the workpiece is relatively thin and the current is adjusted to the low current mode in the general range of 35-50 amperes. The arc extending through the nozzle has a low power density and does not cause high temperature and rapid heating of the nozzle. However, if the inexperienced operator performs the plasma cutting process in the high current mode with current greater than about 50 amperes, the nozzle is grounded by the workpiece. This places the nozzle at the same voltage as the grounded workpiece. When this occurs, it is highly probable that the arc establishing the plasma will actually attach to the nozzle at the outlet walls. This causes an erosion of the nozzle at the outlet. Indeed, since the intent of the plasma is to cut metal, the arc extending through the outlet of the nozzle and attached to the nozzle quickly cuts through the nozzle and destroys the nozzle portion of the torch. In many instances, the operator has absolutely no indication of the reason for the short life of the torch. He replaces the nozzle or the total torch and proceeds in the normal operation. The electrode and nozzle wear with time, since these components are considered to be consumables; however, the use of a plasma cutting torch in a high current mode of operation with the nozzle in contact with the workpiece being cut erodes the nozzle in substantially less time than when the torch is operated properly. This damage to the nozzle is in a substantial order of magnitude faster than should be experienced. Rapid damage to the nozzle causes persons to feel that plasma cutting of metal is not economical. Consequently, in many instances the plasma cutting torch is believed to be too fragile for cutting thick plate, when the problem is not the normal wear of the torch itself, but improper operation of the plasma cutting torch by contacting or dragging the nozzle along the workpiece. The same problem is created in bevel cutting of metal with a plasma torch operated in the high current mode. High arc current is normally required in bevel cutting because of the substantial distance between the workpiece and the outlet of the nozzle. With the torch held at an angle there is a higher probability that the corner of the nozzle will contact the workpiece during bevel cutting. This propensity to contact the workpiece increases the probability of causing the arc to attach to the nozzle. This will destroy the nozzle quite rapidly. Such rapid destruction of the nozzle is more severe in the higher current mode of operation. Also, continuous dragging of the nozzle along the workpiece while operating at high power will damage the torch itself. However, this torch damage occurs slower than nozzle failure.
Consequently, there is substantial adverse market resistance to plasma cutting of metal plates due to the shortness of nozzle life, which is often caused by improper operation of the plasma cutting torch. In the low current mode of operation, i.e. in the neighborhood of about 40 amperes of arc current, the arc does not have sufficient energy to erode the nozzle even if the arc attaches to the nozzle and not the workpiece. The heat is dissipated to keep the nozzle cool. If the arc attaches to the nozzle, which will occur even at an operating current of 40 amperes, the energy density (power) is not nearly as high as in high current operation; consequently, the nozzle life is not substantially shortened.
An operator should know that in the higher current mode of operation a spacing must be maintained between the torch and the workpiece. In addition, the operator should know that if the nozzle is being powered in the high arc current mode when the torch is laid on a grounded plate damage to the nozzle will be caused immediately. Indeed, the nozzle can be melted in this situation. At the lower current operating modes, the arc current is not sufficient to increase the temperature of the nozzle to a melting point temperature. The workpiece or plate will produce a sufficient heat sink to prevent damage to the torch when it is placed on a grounded plate while being operated or driven by power supply in the low arc current mode of operation.